Estimating a Location of a Mobile Device

ABSTRACT

The approximate location of a directed cell of a cellular network is calculated based on locations in the vicinity of which mobile devices were able to detect the directed cell. A mobile device is able to estimate its own location from the approximate locations of one or more directed cells that it can identify. This estimated location of the mobile device may be used to seed its GPS receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/390,214 filed Mar. 28, 2006, entitled “Estimating a Location of aMobile Device”, which is incorporated by reference in its entirety.

BACKGROUND

Information about the location of a mobile device may be required inorder to comply with the Enhanced 911 (E911) directive set by theFederal Communications Commission (FCC). In addition, a location of amobile device may be used in providing location-based services (LBS) toa user of the mobile device.

One method for a mobile device to find its location is the use of GlobalPositioning System (GPS). In another method, denoted Assisted GPS, amobile device may receive a location of a nearby base station and mayoptionally use it for connecting to satellites of the GPS, shorteningthe overall time needed for the process. Cellular network operatorscurrently charge for using the service.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and not limitation in thefigures of the accompanying drawings, in which like reference numeralsindicate corresponding, analogous or similar elements, and in which:

FIG. 1 is an illustration of an exemplary communications system,according to some embodiments of the invention;

FIG. 2 is a block diagram of an exemplary mobile device, according tosome embodiments of the invention;

FIG. 3 is a block diagram of a server in a location approximationsystem, according to some embodiments of the invention.

FIG. 4 shows of exemplary content of a database of a locationapproximation system, according to embodiments of the invention;

FIG. 5 is a flowchart of an exemplary method for a mobile device and fora server of a location approximation system, according to someembodiments of the invention;

FIG. 6 is a flowchart of an exemplary method for a mobile device forcollecting information, according to some embodiments of the invention.

FIG. 7 is a flowchart of an exemplary method in a location approximationsystem, according to some embodiments of the invention;

FIG. 8 shows exemplary directed cell reports stored in a database of alocation approximation system, according to embodiments of theinvention;

FIG. 9 shows exemplary directed cell profiles stored in a database of alocation approximation system, according to embodiments of theinvention;

FIG. 10 is a flowchart of an exemplary method in a locationapproximation system, according to some embodiments of the invention;

FIG. 11 is a block diagram of an exemplary mobile device, according tosome embodiments of the invention; and

FIG. 12 is a flowchart of an exemplary method for a mobile device,according to some embodiments of the invention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some elements may be exaggerated relativeto other elements for clarity.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of embodiments of theinvention. However it will be understood by those of ordinary skill inthe art that the embodiments of the invention may be practiced withoutthese specific details. In other instances, well-known methods,procedures, components and circuits have not been described in detail soas not to obscure the embodiments of the invention.

According to embodiments of the invention, a location approximationsystem is coupled to a cellular network. The location approximationsystem collects information from cellular mobile devices about theirlocations and about the cellular network. The location approximationsystem may be able to approximate the location of a cell based at leastin part on the collected information.

FIG. 1 is an illustration of an exemplary communications system 100,according to some embodiments of the invention. Communications system100 includes a private/public land mobile network (PLMN) 102, a network104, a location approximation system 106 and a GPS satellite system 108.GPS system 108 includes satellites 110 of which only two are shown.

Network 104 may include any combination of wired, wireless and opticalnetworks, and may include additional communication devices (not shown)such as gateways, routers, switches, and the like. Network 104 mayinclude any combination of private networks, public networks and theInternet.

System 106 includes a server 112 and databases 114, 116 and 118, coupledto server 112. PLMN 102 includes a cellular network 122 and aninfrastructure network 124. Cellular network 122 includes base stations126, of which thirteen are shown, that are in communication withinfrastructure 124. System 106 may be able to communicate withinfrastructure 124 via network 104.

A non-exhaustive list of examples for communication standards usedwithin cellular network 122 includes Direct Sequence-Code DivisionMultiple Access (DS-CDMA) cellular radiotelephone communication, GlobalSystem for Mobile Communications (GSM) cellular radiotelephone, NorthAmerican Digital Cellular (NADC) cellular radiotelephone, Time DivisionMultiple Access (TDMA), Extended-TDMA (E-TDMA) cellular radiotelephone,wideband CDMA (WCDMA), General Packet Radio Service (GPRS), EnhancedData for GSM, Evolution (EDGE), 3G and 4G communication.

In system 100, base stations 126 have three directional antennae 128,although this is merely an example and in other communication systemsthe base stations may have any other number of directional antennae.Directional antennae 128 are arranged on base stations 126 to transmitand/or receive signals primarily in sectors of 120°, therefore threedirectional antennae 128 may cover together 360° around any of basestations 126. Any three directional antennae 128 of neighboring basestations 126 that are pointing to a common point form together a logicalunit commonly knows as a cell. The area of a cell is commonly shown as ahexagon, although this is an inaccurate schematic representation of theareas actually covered by the directional antennae forming the cell.

The cell itself is divided into three “directed cells”, each locatednear the closest base station. For example, a cell 130 is divided intodirected cells 132, 134 and 136. Transmissions from directional antennae128 may include unique identifications that identify any particular oneof the directed cells.

Users may carry cellular mobile devices through areas covered bycellular network 122. For example, a mobile device 138 is shown to becarried along a trajectory 140 through cellular network 122.

FIG. 2 is a block diagram of exemplary mobile device 138, according tosome embodiments of the invention. A non-exhaustive list of examples formobile device 138 includes a wireless cellular phone, a wireless-enabledlaptop, a wireless-enabled personal digital assistant (PDA), awireless-enabled smart phone, a wireless-enabled gaming console, and anyother suitable mobile device.

Mobile device 138 includes a processor 200, and a memory 202, a GPSreceiver 204 and a wireless communication interface 206, all coupled toprocessor 200. For clarity, other components of mobile device 138 arenot shown in FIG. 2.

GPS receiver 204 is able to receive signals originating from satellites110, and mobile device 138 may be able to calculate its location frominformation included in the signals.

Communication interface 206 includes at least a baseband controller 208,a radio 210, and an antenna 212 and may be able to communicate with anyof directional antennae 128. If the signals from a directional antennareceived by mobile device 138 are stronger than a threshold, mobiledevice 138 may be able to identify the directed cell and to measurereceived signal strength (RSSI) of the received signals. Possibly,mobile device 138 may also be able to establish communications with theparticular directed cell.

Memory 202 stores code 214 that when executed by processor 202 may causemobile device 138 to perform its part in the methods described in FIGS.5 and 6 below.

Returning to FIG. 1, while at the location shown in a cell 142, mobiledevice 138 may receive signals 144, 146 and 148 that are stronger thanthe threshold from eight directional antennae 128. Mobile device 138 maybe able to identify the eight directed cells of the directional antennaethat generate the signals and to measure RSSI of the signals. As shownin FIG. 2, the identifications 216 of the directed cells and optionallythe RSSI measurements 218 of the signals identifying the directed cellsmay be stored in mobile device 138 in a location report 220 labeled withan identifier 222 of the location.

Many parameters, such as weather and transmission strength, can affectRSSI measurements. However, as a rule of thumb, RSSI measurement of asignal decreases as the distance between the transmitter and thereceiver increases. Therefore, while at the location shown in cell 142,mobile device 138 may measure the strongest RSSI for signal 144, whichoriginates from a directed antenna that is the closest to mobile device138, and may measure the weakest RSSI for signal 148, which originatesfrom a directed antenna that is the farthest (of the eight) from mobiledevice 138.

Other parameters are also proportional to the distance between thetransmitter and the receiver. For example, in some networks, the basestations ask the mobile devices to adjust their transmit power. In suchnetworks, the transmit power of mobile device 138 may be considered anindication of the distance between mobile device 138 and base station126. In another example, a time offset gleaned from the signal may be anindication of the distance between mobile device 138 and base station126. Consequently, location reports 220 may include the transmit powerand/or time offset instead of, or in addition to, the RSSI measurements218.

FIG. 3 is a block diagram of an exemplary server 112, according to someembodiments of the invention. Server 112 includes a processor 300, and amemory 302 and a communication interface 304 coupled to processor 300.For clarity, other components of server 112 are not shown in FIG. 3.Server 112 is able to communicate with network 104 by way of interface304. Memory 302 stores a code 306 that when executed by processor 302may cause server 112 to perform its part in the methods described inFIGS. 5, 7, 10 and 12.

System 106 may attempt to collect information about cellular networkssuch as cellular network 122 from mobile devices such as mobile device138. FIG. 4 shows the exemplary content of database 114, according toembodiments of the invention. Database 114 may include one or moredefinitions 400 of geographical areas of any suitable sizes and shapes.Geographical areas may be defined in any suitable way, for example, bypolygons. For example, a straight line 150 is shown in FIG. 1 to be thecommon boundary of an exemplary geographical area 152 and an exemplarygeographical area 154. Some parts of cellular network 122 are located inarea 152 and other parts of cellular network 122 are located in area154.

Database 114 may optionally include one or more directives 402 and anyof area definitions 400 may be linked to any of directives 402. If, forexample, a particular one of area definitions 400 defines area 154 andis linked to a particular directive 402, the particular directive 402may define, at least in part, how a mobile device that is located withinarea 154 should collect information about cellular network 122.

Directives 402 may define, for example, sampling time intervals formobile devices to sample a cellular network and/or changes in thelocations of mobile devices that should trigger the mobile devices tosample the cellular network. In another example, directives 402 maydefine reporting time intervals for mobile devices to transfer collectedinformation to system 106.

Database 114 may optionally include one or more definitions 404 ofinterest levels and any of area definitions 400 may be linked to any ofinterest levels 404. Interest levels 404 may store numerical values orany other suitable representations of the levels of interest of system106 in receiving information originating from geographical areas. Anyparticular one of area definitions 400 may be linked to a particulardirective 402 or a particular interest level 404.

FIG. 5 is a flowchart of an exemplary method for a mobile device, forexample, mobile device 138, and for server 112, according to someembodiments of the invention. At 500, mobile device 138 determines itscurrent location, for example, by using GPS receiver 204.

At 502, mobile device 138 checks whether memory 202 stores validinstructions for data collection at the vicinity of its location. Asshown in FIG. 2, memory 202 may store one or more area definitions 224and one or more directives 226. If a particular area definition 224defines an area in which mobile device 138 is currently located and adirective 226 linked to that particular definition 224 includes validcollection instructions, the method may continue to 520.

Otherwise, at 504, mobile device 138 reports its current location tosystem 106 and at 506, server 112 may receive the report. At 508, server112 may retrieve from database 114 a particular area definition 400 thatdefines an area including the current location of mobile device 138. Forexample, if the current location of mobile device 138 is as shown inFIG. 1 in a cell 156, server 112 may retrieve an area definition 400 forarea 152. In addition, server 112 may optionally retrieve other areadefinitions 400 that define other areas of interest, for example, areasin the vicinity of area 152, e.g. area 154.

At 510, server 112 may retrieve from database 114 directives 402 and/orinterest levels 404 that are linked to the retrieved area definitions400. At 512, server 112 may transmit the retrieved information to mobiledevice 138 and at 514, mobile device 138 may receive the information.

At 516, mobile device 138 may store directives 400 and interest levels404 received at 514 in fields 224 and 226, respectively in memory 202(shown in FIG. 2).

A dictionary 228 (shown in FIG. 2) may be stored in memory 202.Dictionary 228 may receive as input any of interest levels 404 and mayoutput corresponding instructions similar to directives 402. If mobiledevice 138 receives at 514 any of interest levels 404, at 518, device138 may use dictionary 228 to identify corresponding directives and maystore those directives in fields 226. At 520, mobile device 138initiates collection of information and the method terminates.

FIG. 6 is a flowchart of an exemplary method for a mobile device, forexample mobile device 138, for collecting information, according to someembodiments of the invention. At 600, mobile device 138 determines itscurrent location. If memory 202 does not store a definition 224 of anarea containing the location determined at 600 (checked at 602), or ifmemory 202 does not store valid directives 226 for that area (checked at604), the method may proceed to step 504 of FIG. 5. For example, adirective 226 may not be valid if it has an expired time tag.

Otherwise, at 610 and according to area definition 224 and directives226 recognized at 602 and 604, respectively, while in the vicinity ofthe determined location, mobile device 138 collects information fromsignals it receives from directional antennae. Mobile device 138 maystore the information collected at 600 and 610 as one location report220 in memory 202.

At 612, mobile device 138 checks whether, according to the directives226 for the area in which the current mobile device 138 is located, itis time for device 138 to report location reports 220 to system 106. Ifso, then at 614 mobile device 138 may transmit location reports 220 thatare stored in memory 202 to system 106. Device 138 may wait at 616until, according to the directives for the area in which the currentmobile device 138 is located, it is time for mobile device 138 tocollect more information and the method may continue to 600.

FIG. 7 is a flowchart of an exemplary method in system 106, according tosome embodiments of the invention.

Many mobile devices, such as mobile device 138, may collect locationreports 220, and may send several location reports 220 to system 106. At700, server 112 may receive a particular location report 220 from mobiledevice 138. Server 112 may decide at 702 not to incorporate theparticular location report 220 into database 116 and the method mayterminate. Otherwise, at 704, server 112 may incorporate informationincluded in the particular location report 220 into one or more ofdirected cell reports, and may optionally discard from database 116information previously incorporated from other location reports.

Information in selected location reports 220 received by system 106 fromvarious mobile devices is collated into directed cell reports stored indatabase 116. FIG. 8 shows exemplary directed cell reports stored indatabase 116, according to embodiments of the invention. A directed cellreport 800 identifies the directed cell for which it containsinformation with identification 216. Directed cell report 800 alsoincludes those location identifiers 222 of location reports 220 thatincluded directed cell ID 216. Optionally, directed cell report 800 alsoincludes RSSI measurements 218 for the directed cell identified by ID216 obtained from location reports 220 that included directed cell ID216. In other embodiments, directed cell report 800 includes transmitpowers and/or time offsets instead of or in addition to the RSSImeasurements.

A location report can therefore be understood as identifying thedirected cells whose signals were received by mobile devices when in thelocation identified by the location ID of the location report. Thelocation report may also include RSSI measurements of the signals, timeoffsets gleaned from the signals and/or transmit powers of the mobiledevices when in the location identified by the location ID of thelocation report.

A directed cell report can be understood as identifying the locations atwhich mobile devices were able to identify received signals asoriginating in the directed cell. The directed cell report may alsoinclude RSSI measurements of those received signals, time offsetsgleaned from the signals, and/or transmit powers of the mobile devices.

FIG. 9 shows exemplary directed cell profiles 900 in database 118,according to embodiments of the invention. FIG. 10 is a flowchart of anexemplary method in system 106, according to some embodiments of theinvention.

Any of directed cell reports 800 may include a vast amount of locations222 and optionally RSSI measurements 218, for example few hundreds totenth of thousands. Once enough information has been stored in aparticular directed cell report 800, or when the particular directedcell report 800 has been updated, or for any other reason, server 112may create or update a directed cell profile 900 in database 118 thathas the same directed cell ID 216.

At 1004, a server 112 calculates the approximate location of thedirected cell identified by directed cell ID 216 from at least locations222 in the corresponding directed cell report 800. For example, server112 may calculate the median or average of locations 222, separately foreach axis (e.g. longitude and latitude). Server 112 may store theapproximate location in a field 902 in the particular directed cellprofile 900. It should be noted that the approximate location of aparticular directed cell calculated by server 112 from locations 222 inthe corresponding directed cell report 800 is likely different than thelocation of base station 126 a directional antenna of which transmitssignals for the particular directed cell.

At 1006, server 112 may optionally store the maximal one of RSSImeasurements 218 in a field 904 in the particular directed cell profile900. At 1008, server 112 may optionally calculate a statisticalcharacteristic, for example, standard deviation, of RSSI measurements218 and may store the result in a field 906 in the particular directedcell profile 900. In other embodiments, server 112 may optionally storethe maximal transmit power and/or time offset and/or statisticalcharacteristics thereof in the particular directed cell profile.

FIG. 11 is a block diagram of an exemplary mobile device 1100, accordingto some embodiments of the invention. FIG. 11 is similar to FIG. 2except that code 1106 stored in memory 202 implements its part of themethod described in FIG. 12. In addition, mobile device 1100 is notnecessarily equipped with GPS receiver 204.

FIG. 12 is a flowchart of an exemplary method for mobile device 1100 andserver 112, according to some embodiments of the invention. As shown inFIG. 1, device 1100 is located in a cell 158 in area 152. At 1200,device 1100 collects identifications of one or more directed cells inits vicinity. These directed cells are identified from signals receivedby device 1100 from directional antennae 128 in cellular network 122.Device 1100 may also measure the RSSI of those signals. In otherembodiments, device 1100 may glean time offsets of those signals and/ornote its transmit power at the time the signals were received. At 1202,device 1100 may transmit to system 106 a query for positions of theidentified directed cells, and server 112 may receive the query at 1204.

At 1206, server 112 may retrieve from database 118 one or more directedcell profiles 900 corresponding to the directed cells identified in thequery. At 1208, server 112 may transmit the retrieved directed cellprofiles 900 and at 1210, device 1100 receives the transmission. Theinformation received at 1210 may be cached by device 1100 for futurereference, thus circumventing the need to request the locations ofdirected cells more than once.

At 1212, device 1100 may optionally estimate its location from theinformation received at 1210. For example, if the query is for theposition of a single directed cell, then device 1100 may estimate itslocation as the approximate location of the directed cell received fromserver 112. If the query is for the positions of two or more directedcells, then device 1100 may estimate its location for each axis as theweighted average of the coordinates of each directed cell, where theweight function is a linear mapping of signal strength from eachdirected cell. For example, if the query is for the positions ofdirected cells A, B and C, and device 1100 has measured RSSI valuesRSSI_(A), RSSI_(B) and RSSI_(C) respectively for signals identifyingthose directed cells, and the positions received from server 112 are inlatitude and longitude coordinates, then the calculation is as follows:

${{device}\mspace{14mu} {lat}} = \frac{{{w\left( {RSSI}_{A} \right)} \cdot {lat}_{A}} + {{w\left( {RSSI}_{B} \right)} \cdot {lat}_{B}} + {{w\left( {RSSI}_{C} \right)} \cdot {lat}_{c}}}{{w\left( {RSSI}_{A} \right)} + {w\left( {RSSI}_{B} \right)} + {w\left( {RSSI}_{C} \right)}}$${{device}\mspace{14mu} {long}} = \frac{{{w\left( {RSSI}_{A} \right)} \cdot {long}_{A}} + {{w\left( {RSSI}_{B} \right)} \cdot {long}_{B}} + {{w\left( {RSSI}_{C} \right)} \cdot {long}_{c}}}{{w\left( {RSSI}_{A} \right)} + {w\left( {RSSI}_{B} \right)} + {w\left( {RSSI}_{C} \right)}}$

where w is the weight function. In other embodiments, the weightfunction is a linear mapping of transmit power and/or time offset.

At 1214, if equipped with GPS receiver 204, device 1100 may optionallyuse its approximated location to communicate with satellites 110. Thismay shorten the time required for GPS receiver 204 to determine a moreprecise location of device 1100.

Many modifications will occur to persons of ordinary skill in the art.For example, databases 114, 116 and 118 may be a single database ordistributed databases. In another example, separate servers maysubstitute for server 112.

It will be appreciated by persons of ordinary skill in the art how tomodify the foregoing to apply to other radio frequency (RF)communication systems. The term “directed cell” may therefore beunderstood to include any transmit area from any uniquely identifiableRF source. For example, if omnidirectional antennae are substituted forthe directional antennae described hereinabove, then the directed cellis understood to be the entire cell. In another example, if thecommunication system is an IEEE 802.11 WLAN system and access points aresubstituted for the base stations described hereinabove, then thedirected cell is understood to be the coverage area of the access point.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

1. A method for estimating a location of a mobile device in a network,the method comprising: identifying two or more uniquely identifiableradio frequency sources of the network from signals received by themobile device at the location; receiving approximate locations of two ormore transmit areas from the uniquely identifiable radio frequencysources; determining for each signal received by the mobile device oneor more parameters; and estimating the location of the mobile devicefrom the approximate locations and the one or more parameters.
 2. Themethod of claim 1, wherein at least one of the parameters is a maximumsignal strength measurement ‘RSSI’ of one of the signals.
 3. The methodof claim 2, wherein the approximate locations are provided ascoordinates on axes and estimating the location from the approximatelocations and the one or more parameters comprises: estimating thelocation as a weighted average of the coordinates on each of the axes,where weights used in calculating the weighted average are linearfunctions of signal strength measurements of signals from which themobile device identified the uniquely identifiable radio frequencysources.
 4. The method of claim 1, wherein at least one of theparameters is a transmit power of the mobile device at the time one ofthe signals was received.
 5. The method of claim 1, wherein at least oneof the parameters is a time offset gleaned from one of the signals. 6.The method of claim 1, wherein the network is a cellular network, theuniquely identifiable radio frequency sources are base stations of thecellular network having directional antennae, and the transmit areas aredirected cells of the cellular network.
 7. The method of claim 1,wherein the network is a cellular network, the uniquely identifiableradio frequency sources are base stations of the cellular network havingomnidirectional antennae, and the transmit areas are entire cells of thecellular network.
 8. The method of claim 1, wherein the network is awireless local area network, the uniquely identifiable radio frequencysources are access points of the wireless local area network, and thetransmit areas are the coverage areas of the access points.
 9. A methodfor assisting a mobile device to estimate its location in a network, themethod comprising: receiving from mobile devices location reports thatinclude locations of the mobile devices, an identification of a uniquelyidentifiable radio frequency source in the network identified by themobile devices at the locations, and transmit powers of the mobiledevices to the uniquely identifiable radio frequency source at thelocations; constructing from the location reports a radio frequencysource profile for the uniquely identifiable radio frequency source, theradio frequency source profile including information based on thetransmit powers and including an approximate location of a transmit areafor the uniquely identifiable radio frequency source; receiving from themobile device a query that identifies the uniquely identifiable radiofrequency source; and in response to the query, transmitting the radiofrequency source profile to the mobile device to enable the mobiledevice to use the approximate location and the information in estimatingits location.
 10. The method of claim 9, wherein the informationcomprises a maximal transmit power of the transmit powers.
 11. Themethod of claim 9, wherein the information comprises a statisticalcharacteristic of the transmit powers.
 12. The method of claim 9,further comprising: determining the approximate location as a set ofcoordinates on perpendicular axes, wherein the set of coordinatesrepresents statistical characteristics of the locations.
 13. The methodof claim 9, wherein the network is a cellular network, the uniquelyidentifiable radio frequency source is a base station of the cellularnetwork having directional antennae, and the transmit area is a directedcell of the cellular network.
 14. The method of claim 9, wherein thenetwork is a cellular network, the uniquely identifiable radio frequencysource is a base station of the cellular network having anomnidirectional antenna, and the transmit area is an entire cell of thecellular network.
 15. The method of claim 9, wherein the network is awireless local area network, the uniquely identifiable radio frequencysource is an access point of the wireless local area network, and thetransmit area is a coverage area of the access point.
 16. A mobiledevice comprising: a communication interface through which the mobiledevice is able to receive signals from one or more uniquely identifiableradio frequency sources of a network; a processor coupled to thecommunication interface; and a memory coupled to the processor, thememory arranged to store executable code which, when executed by theprocessor, is arranged to identify two or more uniquely identifiableradio frequency sources of the network from signals received by themobile device at the location, receive approximate locations of two ormore transmit areas from the uniquely identifiable radio frequencysources, determine for each signal received by the mobile device one ormore parameters, and estimate the location of the mobile device from theapproximate locations and the one or more parameters.
 17. A locationapproximation system comprising: a database to store a radio frequencysource profile for a uniquely identifiable radio frequency source of afirst network, the radio frequency source profile including anapproximate location of a transmit area for the uniquely identifiableradio frequency source, wherein the uniquely identifiable radiofrequency source is able to communicate with an infrastructure network;a server coupled to the database; and means for communicating with theinfrastructure network, wherein the server is arranged to receive frommobile devices location reports that include locations of the mobiledevices, an identification of the uniquely identifiable radio frequencysource identified by the mobile devices at the locations, and transmitpowers of the mobile devices to the uniquely identifiable radiofrequency source at the locations, wherein the server is furtherarranged to construct the radio frequency source profile from thelocation reports, wherein the radio frequency source profile includesinformation related to the transmit powers, and wherein, in response toreceipt by the server of a query from a mobile device identifying theuniquely identifiable radio frequency source, the server is arranged totransmit the radio frequency source profile to the mobile device toenable the mobile device to use the approximate location and theinformation in estimating its location.
 18. The location approximationsystem of claim 17, wherein the information comprises a maximal transmitpower of the transmit powers.
 19. The location approximation system ofclaim 17, wherein the information comprises a statistical characteristicof the transmit powers.
 20. The location approximation system of claim17, wherein the server is further adapted to determine the approximatelocation as a set of coordinates on perpendicular axes, wherein the setof coordinates represents statistical characteristics of the locations.21. The location approximation system of claim 17, wherein the networkis a cellular network, the uniquely identifiable radio frequency sourceis a base station of the cellular network having directional antennae,and the transmit area is a directed cell of the cellular network. 22.The location approximation system of claim 17, wherein the network is acellular network, the uniquely identifiable radio frequency source is abase station of the cellular network having an omnidirectional antenna,and the transmit area is an entire cell of the cellular network.
 23. Thelocation approximation system of claim 17, wherein the network is awireless local area network, the uniquely identifiable radio frequencysource is an access point of the wireless local area network, and thetransmit area is a coverage area of the access point.